Investigators have recently turned their attention to understanding the biological mechanisms of taste, and in particular bitter taste. For a review of the literature see, for example, Science 291, 1557-1560. (2001); Cell 100, 607-610 (2000); Neuron 25, 507-510 (2000); Nature 413, 219-225. (2001); and J. Biol. Chem. 277, 1-4 (2001).
Bitter taste is aversive, and as such provides humans with a mechanism of protection against poisonous substances, which are generally bitter-tasting compounds. More subtly, bitter-tastants also affect the palatability of food, beverages, thereby influencing human nutritional habits as is more fully discussed by Drewnowski in “The Science and Complexity of Bitter Taste”, Nutr. Rev. 59, 163-169 (2001). They also affect the palatability of other ingestibles such as orally administered pharmaceuticals and nutraceuticals. Understanding the mechanism of bitter taste transduction has implications for the food and pharmaceutical industries. If the bitter taste transduction pathway can be manipulated, it may be possible to suppress or eliminate bitter taste to render foods more palatable and increase patient compliance in oral pharmaceutics.
Taste transduction involves the interaction of molecules, i.e., tastants with taste receptor-expressing cells which reside in the taste buds located in the papillae of the tongue. Taste buds relay information to the brain on the nutrient content of food and the presence of poisons. Recent advances in biochemical and physiological studies have enabled researchers to conclude that bitter taste transduction is mediated by so-called G-protein coupled receptors (GPCRs). GPCRs are 7 transmembrane domain cell surface proteins that amplify signals generated at a cell surface when the receptor interacts with a ligand (a tastant) whereupon they activate heterotrimeric G-proteins. The G-proteins are protein complexes that are composed of alpha and beta-gamma subunits. They are usually referred to by their alpha subunits and classified generally into 4 groups: Galpha s, i, q and 12. The Galpha q type couple with GPCRs to activate phospholipase C which leads to the increase in cellular Ca2+. There are many Gq-type G-proteins that are promiscuous and can couple to GPCRs, including taste receptors, and these so-called “promiscuous” G-proteins are well known to the man skilled in the art. These G-proteins dissociate into alpha and beta-gamma subunits upon activation, resulting in a complex cascade of cellular events that results in the cell producing cell messengers, such as calcium ions, that enable the cells to send a signal to the brain indicating a bitter response.
There is also anatomical evidence that GPCRs mediate bitter taste transduction: clusters of these receptors are found in mammalian taste cells containing gustducin. Gustducin is a G-protein subunit that is implicated in the perception of bitter taste in mammals, see for example Chandrashekar, J. et al., Cell 100, 703-711 (2000); Matsunami H. et al., Nature 404, 601-604 (2000); or Adler E. et al., Cell 100, 693-702 (2000). cDNAs encoding such GPCRs have been identified, isolated, and used as templates to compare with DNA libraries using in-silico data-mining techniques to identify other related receptors. In this manner it has been possible to identify a family of related receptors, the so-called T2R family of receptors, that have been putatively assigned as bitter receptors.
To-date, however, it is not clear as to whether all the bitter taste receptors have been discovered. Further, of those that have been discovered, many have not been matched, or paired, with ligands, and applicant is aware of very few published studies wherein rigorous matching has been undertaken. Chandrashekar, J. et al. in Cell 100, 703-711 (2000), has expressed a human T2R receptor, the so-called hT2R4 receptor, in heterologous systems and looked at the in vitro response of this receptor. They found that it provided a response to the bitter compounds denatonium and 6-n-propyl-2-thiouracil. However, the concentrations of bitter tastants needed to activate the hT2R4 receptor were two orders of magnitude higher than the thresholds reported in human taste studies, and so it is not clear that the protein encoded by the hT2R4 gene is a functional bitter receptor. The authors of the Chandrashekar et al. article also looked at a number of mouse T2R receptors with a range of stock bitter-tasting chemicals of disparate chemical structure. However, no study has looked at receptor responses to bitter ligands that are problematic in the food and pharmaceutical industries, and means of suppressing the bitter response to these ligands.
The universe of compounds that provoke a bitter response in humans is structurally very diverse. Therefore, if research into bitter receptors is to be of any practical significance to the food and pharmaceutical industries, all bitter receptors will need to be identified, and once identified, there has to be a rigorous understanding of how specific receptors are matched to particular structural classes of bitter compounds. Unfortunately, although much basic research has been conducted in the area of bitter taste receptors, there are potentially many more bitter receptors to be discovered, and little is still known as to whether the known members of the human T2R family of bitter receptors actually respond to bitter tastants, and if so what, if any, specificity they show to ligand substructures.